The invention deals with the field of materials science. It relates to a nickel-base superalloy, in particular for the production of single-crystal components, such as blades or vanes for gas turbines.
Nickel-base superalloys of this type are known. Single-crystal components made from these alloys have a very good strength at high temperatures. This allows, for example, the intake temperature of gas turbines to be increased, with the result that the efficiency of the gas turbine rises.
Nickel-base superalloys for single-crystal components, as are known from U.S. Pat. No. 4,643,782, EP 0 208 645 and U.S. Pat No. 5,270,123, for this purpose contain alloying elements which strengthen the solid solution, for example Re, W, Mo, Co, Cr, and elements which form xcex3xe2x80x2 phases, for example Al, Ta and Ti. The level of high-melting alloying elements (W, Mo, Re) in the basic matrix (austenitic xcex3 phase) increases continuously as the temperature of load on the alloy increases. For example, standard nickel-base superalloys for single crystals contain 6-8% of W, up to 6% of Re and up to 2% of Mo (details in % by weight). The alloys disclosed in the abovementioned documents have a high creep strength, good LCF (low cycle fatigue) and HCF (high cycle fatigue) properties and a high resistance to oxidation.
These known alloys were developed for aircraft turbines and were therefore optimized for short-term and medium-term use, i.e. the load time was designed for up to 20,000 hours. By contrast, industrial gas turbine components have to be designed for a load time of up to 75,000 hours.
By way of example, after a load time of 300 hours, the alloy CMSX-4 which is known from U.S. Pat. No. 4,643,782, when it was tested for use in a gas turbine at a temperature of over 1000xc2x0 C., underwent considerable coarsening of the xcex3xe2x80x2 phase, which disadvantageously leads to an increase in the creep rate of the alloy.
The alloys which are known, for example, from U.S. Pat. No. 5,270,123 also have similar drawbacks. The alloying elements selected in that document cause, in the abovementioned alloys, a positive or negative lattice offset between the xcex3 phase which forms the matrix and the xcex3xe2x80x2 phase, i.e. the secondary intermetallic phase Ni3Al, in which Ta, Ti, Hf may partially replace Al and Co and Cr may partially replace Ni. This lattice strain prevents dislocations during sliding or cutting of the xcex3xe2x80x2 grains. Although the lattice strain increases the short-term strength, under longer load the microstructure becomes coarser, followed by degradation of the xcex3xe2x80x2 structure, with an associated long-term mechanical weakening of the alloy.
This drawback is eliminated by the alloy which is known from EP 0 914 483 B 1. This nickel-base superalloy essentially consists of (measured in % by weight) 6.0-6.8% Cr, 8.0-10.0% Co, 0.5-0.7% Mo, 6.2-6.6% W, 2.7-3.2% Re, 5.4-5.8% Al, 0.5-0.9Ta, 0.15-0.3% Hf, 0.02-0.04% C, 40-100 ppm B, 0-400 ppm Y, remainder Ni with impurities, where the ratio of (Ta+1.5 Hf+0.5 Moxe2x88x920.5 Ti)/(W+1.2 Re) is xe2x89xa70.7. On account of the abovementioned ratio of the alloying elements, at operating temperature these alloys do not have a lattice offset between the xcex3 phase and the xcex3xe2x80x2 phase, with the result that a high long-term stability under moderate load is achieved. In addition, this rhenium-alloyed nickel-base superalloy has excellent casting properties and a good phase stability in combination with optimum mechanical properties. Moreover, it is distinguished by a high fatigue strength and creep stability even under long-term load.
Furthermore, it has been determined that in the presence of a mechanical load and a long-term high-temperature stress, there is targeted coarsening of the xcex3xe2x80x2 particles, the phenomenon known as rafting, and at high xcex3xe2x80x2 contents, (i.e. at a xcex3xe2x80x2 content of at least 50% by volume) the microstructure is inverted, i.e. xcex3xe2x80x2 becomes the continuous phase in which what was previously the xcex3 matrix is embedded. Since the intermetallic xcex3xe2x80x2 phase tends toward environmental embrittlement, under certain loading conditions this leads to a massive drop in the mechanical properties, in particular the yield strength, at room temperature (degradation of the properties). Environmental embrittlement occurs in particular in the presence of moisture and long holding times under tensile load.
It is an object of the invention to avoid the abovementioned drawbacks. The invention is based on the object of developing a nickel-base superalloy which, on the one hand, has a solid, strong xcex3 phase as the matrix and, on the other hand, has only a low level, i.e. less than 50%, of xcex3xe2x80x2 phase, and is therefore very resistant to oxidation and has a good creep behavior.
According to the invention, this object is achieved by the fact that the nickel-base superalloy according to the invention is characterized by the following chemical composition (details in % by weight):
7-13 Cr
4-10 Co
0.5-2 Mo
2-8 W
4-6 Ta
3-6 Al
1-4 Ti
0.1-6 Ru
0.01-0.5 Hf
0.001-0.15 Si
0-700 ppm C
0-300 ppm B
remainder nickel and production-related impurities.
The advantages of the invention consist in the fact that the alloy has a good degradation behavior. The xcex3 phase (matrix) is strengthened by the addition of ruthenium to the alloy, despite the absence of rhenium, which according to the known prior art is considered to be a particularly good element for strengthening the solid solution and therefore greatly improves the properties of the xcex3 matrix. The alloy according to the invention is distinguished by good creep rupture strength, a stable microstructure and good casting properties.
Moreover, the resistance of the alloy to oxidation is very good. The alloy is eminently suitable for the production of single-crystal components, for example blades or vanes for gas turbines.
On account of the low level of secondary precipitation-hardening xcex3xe2x80x2 phase which is incorporated in the greatly strengthened xcex3 phase, the degradation behavior of the alloy according to the invention is good. There is no single-crystal crack growth and no great drop in the yield strength at room temperature in the degraded state compared to the undegraded state.
Preferred ranges for the nickel-base superalloy according to the invention are (details in % by weight):
10-13 Cr
8-9 Co
1.5-2 Mo
3-5 W
4-5 Ta
3-5 Al
2-4 Ti
0.3-4 Ru
0.01-0.5 Hf
0.001-0.15 Si
0-700 ppm C
0-300 ppm B
remainder nickel and production-related impurities.
A particularly preferred range for the nickel-base superalloy according to the invention is as follows:
10-13 Cr
8-9 Co
1.5-2 Mo
3.5-4 W
4-5 Ta
3.5-5 Al
3-4 Ti
0.3-1.5 Ru
0.5 Hf
10-500 ppm Si
250-350 ppm C
80-100 ppm B
remainder nickel and production-related impurities.
A further nickel-base superalloy according to the invention has the following chemical composition (details in % by weight):
7-9 Cr
8-9 Co
1.5-2 Mo
3-5 W
5-6 Ta
3-5 Al
1-2 Ti
0.5-1.5 Ru
0.5 Hf
700 ppm C
100 ppm B
500 ppm Si
remainder nickel and production-related impurities.